configs: am64: Fix booting of fitImage on AM64x

Do not limit the maximum size of the buffer that is used to decompress
the OS image in to, this causes issue while inflating the image, if image
size is greater than the buffer.
Remove CONFIG_SYS_BOOTM_LEN

Signed-off-by: Kamlesh Gurudasani <kamlesh@ti.com>
Signed-off-by: Manorit Chawdhry <m-chawdhry@ti.com>